Global distribution of ship tracks from one year of AATSRdata

Abstract

The perturbation of a cloud layer by ship-generated aerosol changes the cloud reflectivity and is identified by elongated structures in satellite images, known as ship tracks. As ship tracks indicate a pollution of the clean marine environment and also affect the radiation budget below and above the cloud, it is important to investigate their radiative and climate impact. In this study we use satellite data to examine the effects of ship tracks on a particular scene as well as on the global scale. The cloud optical and microphysical properties are derived using a semi-analytical retrieval technique combined with a look-up-table approach. Within the ship tracks a significant change in the droplet number concentration, the effective radius and the optical thickness are found compared to the unaffected cloud. The resulting cloud properties are used to calculate the radiation budget below and above the cloud. Local impacts are shown for a selected scene from MODIS on Terra. The mean reflectance at top of atmosphere (TOA) is increased by 40.8 Wm-2. For a particular scene chosen close to the West Coast of North America on 10th February 2003, ship emissions increase the backscattered solar radiation at TOA by 2.0Wm-2, corresponding to a negative radiative forcing (RF). A global distribution of ship tracks derived from one year of AATSR data shows high spatial and temporal variability with highest occurrence of ship tracks westward of North America and the southwest coast of Africa, but small RF on the global scale.